(1) Field of the Invention
The present invention relates to a so called "tandem-type" image forming apparatus which is provided with a plurality of image forming units and which is capable of operating in a full-color print mode and a single-color print mode. All of the image forming units are operative in the full-color print mode to form a color image, while only one of the image forming units is operative in the single-color mode to form a single-color image.
(2) Related Art
A tandem-type image forming apparatus, as one example of a conventional full-color image forming apparatus, has image forming units corresponding to toner colors set in line. A transport means of a tandem-type image forming apparatus transports a recording sheet to each transfer position (i.e., a position at which a toner image is transferred onto the recording sheet) of the image forming units. Color toner images formed by the image forming units are sequentially superimposed to form a full-color image on the recording sheet.
In general, a tandem-type image forming apparatus has four image forming units corresponding to four colors, that is, cyan (C), magenta (M), yellow (Y), and black (K). The image forming unit used for forming a black image is usually set at a rearmost position of a transportation path of the recording sheet transported by the transport means to prevent blurring of colors. An attraction charger is set at a frontmost position of the transportation path of the recording sheet and charges the recording sheet supplied from a sheet supply unit so that it is attracted to the transport means. The recording sheet is attracted to the transport means by the electrostatic charge of the attraction charger and sequentially transported to each transfer position of the image forming units without moving away from a correct position on the transport means. As a result, the multicolor image can be formed without color displacement.
A transport belt can be used as the transport means. In general, a transport belt is made of a semi-conductive material to encourage the self-elimination of static. The surface potential of the transport belt gradually decreases with time after the transport belt has been charged. For this reason, the electrostatic force which attracts the recording sheet decreases as the recording sheet is transported in a transport direction thereof. However, when a full-color image is formed, the recording sheet and the transport belt are charged with transfer voltage at the transfer positions of the image forming units for cyan, magenta, yellow, and black in addition to being charged by the attraction charger. Consequently, the recording sheet can be reliably transported all of the way to a fixing device, with the attraction being maintained above a predetermined level.
When only one of the image forming units is used, however, such as when forming a black image using only the final image forming unit, the recording sheet may move away from the correct position on the transport means before reaching the transfer position of the image forming unit used for forming the black image since the electrostatic force applied to the transport means decreases due to the self-elimination of static. Alternatively, a paper jam may occur, depending on the type of the recording sheet being used and the conditions inside the image forming apparatus.
Japanese Laid-Open Patent Application No. 4-221982 teaches an example of a tandem-type image forming apparatus which has all of the image forming units apply a transfer voltage even when the image is formed using only some of the image forming units. Thus, the electrostatic force of the recording sheet attracted to the transport means can be sufficiently maintained even when the image is formed using not all of the image forming units, so that the recording sheet does not move away from the correct position on the transport means.
General speaking, however, in organizations such as offices, the number of image formations using only some of the image forming units, such as operations using only the image forming unit for forming a black image, will be considerably higher than the number of image formations using all of the image forming units for forming color images. As in the cited Japanese application, if electrostatic charge is applied the same number of times, regardless of whether the single-color image formation or the full-color image formation, there will be an increase in power consumption in addition to a great acceleration in the wear and tear on the transport means.
Tandem-type image forming apparatuses are usually provided with a variety of modes in addition to the ability to select between the color image formation and the single-color image formation. If electrostatic charge is applied to the recording sheet to be attracted to the transport means the same number of times, regardless of the current mode, there will be an increase in power consumption in addition to a great acceleration in the wear and tear on the transport means.
The tandem-type image forming apparatus operates based on the premise that high-quality paper is used as the recording sheet for better reproduction of color images. By using high-quality paper, the attraction of the recording sheet relative to the transport means is ensured. FIG. 1 is a graph which compares the attraction of high-quality paper, with those of recycled paper and low-grade paper which have rough surfaces.
As shown in the graph, the attraction of recycled paper and low-grade paper are extremely low, compared with the attraction of high-quality paper. As such, when recycled paper or the like is used as the recording sheet in the conventional tandem-type image forming apparatus, the recording sheet is not reliably transported, and so is skewed or moves away from the correct position on the transport means. At worst, the transport means cannot transport the recording sheet, causing a paper jam.
However, there are increasing user demands for the use of recycled paper or other low-grade paper as the recording sheet to reduce costs, since it is not necessary to consider the prerequisites for color reproduction when reproducing a single-color image.